Not Applicable.
The present invention relates to a process for the catalytic partial oxidation of hydrocarbons (e.g., natural gas), and in particular to a process for oxidizing methane to produce a mixture of carbon monoxide and hydrogen using a bulk rhodium catalyst in the form of porous foam or monolith.
Large quantities of methane, the main component of natural gas, are available in many areas of the world, and natural gas is predicted to outlast oil reserves by a significant margin. However, most natural gas is situated in areas that are geographically remote from population and industrial centers. The costs of compression, transportation, and storage make its use economically unattractive.
To improve the economics of natural gas use, much research has focused on methane as a starting material for the production of higher hydrocarbons and hydrocarbon liquids. The conversion of methane to hydrocarbons is typically carried out in two steps. In the first step, methane is converted into a mixture of carbon monoxide and hydrogen (i.e., synthesis gas or syngas). In a second step, the syngas is converted into hydrocarbons.
Current industrial use of methane as a chemical feedstock proceeds by the initial conversion of methane to carbon monoxide and hydrogen by either steam reforming, which is the most widespread process, or by dry reforming. Steam reforming is currently the major process used commercially for the conversion of methane to synthesis gas, and proceeds according to Equation 1.
CH4+H2O⇄CO+3H2xe2x80x83xe2x80x83(1)
Although steam reforming has been practiced for over five decades, efforts to improve the energy efficiency and reduce the capital investment required for this technology continue.
The catalytic partial oxidation of hydrocarbons, e.g., natural gas or methane to syngas is also a process known in the art. While currently limited as an industrial process, partial oxidation has recently attracted much attention due to its significant inherent advantages, such as the fact that significant heat is released during the process, in contrast to steam reforming processes.
In catalytic partial oxidation, natural gas is mixed with air, oxygen-enriched air, or oxygen, and introduced to a catalyst at elevated temperature and pressure. The partial oxidation of methane yields a syngas mixture with a H2:CO ratio of 2:1, as shown in Equation 2.
CH4+1/2O2⇄CO+2H2xe2x80x83xe2x80x83(2)
This ratio is more useful for the downstream conversion of the syngas to chemicals such as methanol and to fuels than the H2:CO ratio steam reforming. The partial oxidation is also exothermic, while the steam reforming reaction is strongly endothermic. Furthermore, oxidation reactions are typically much faster than reforming reactions. This allows the use of much smaller reactors for catalytic partial oxidation processes.
The selectivities of catalytic partial oxidation to the desired products, carbon monoxide and hydrogen, are controlled by several factors, but one of the most important of these factors is the choice of catalyst structure. For successful operation on a commercial scale, the catalytic partial oxidation process must be able to achieve a high conversion of the methane feedstock at high gas hourly space velocities, while maintaining high selectivity of the process to the desired products of carbon monoxide and hydrogen. Accordingly, there has been an effort to investigate catalysts that provide high selectivity for specified products and have structures that promote partial oxidation of hydrocarbons at high gas hourly space velocities, such as foam monolithic catalysts. Monolithic catalysts, such as foam, have certain advantages as compared to particulate catalysts, such as sponge. These include high reactant throughput, low catalyst cost, reduced reactor size, and ease of replacement.
U.S. Pat. No. 5,648,582 discloses a process for the catalytic partial oxidation of methane in gas phase at very short residence time (a space velocity of 800,000 to 12,000,000 hrxe2x88x921) by contacting a gas stream containing methane and oxygen with a metal supported catalyst, such as platinum, rhodium, or nickel deposited on a monolith. According to the ""582 patent, ceramic foam monoliths are preferred where hydrogen production is the desired process use of the synthesis gas. The preferred metals content comprises rhodium or nickel loadings on the monoliths of 1 to 15 percent as applied by washcoats.
European Patent Application EP 0576096 discloses a process for the catalytic partial oxidation of a hydrocarbon feedstock by contacting the feedstock and an oxygen-containing gas with a metal selected from Group VII of the Periodic Table supported on a carrier, in a fixed arrangement having high tortuosity. The fixed arrangement of the catalyst is disclosed to be a fixed bed of a particulate catalyst or a ceramic foam. Rhodium and platinum catalysts on an alpha-alumina support were tested. The rhodium catalyst exhibited a greater intrinsic activity in the catalytic partial oxidation reactions than did the platinum catalyst.
In each of the above-disclosed processes, the catalyst is required to be supported on a carrier. Supported catalysts can be less resistant to thermal shock and may be subject to undesired interactions between the catalytic metal and the material of the carrier. Further, supported catalysts have the disadvantage of hindering reclamation of the catalyst material once the catalyst lifetime has been exceeded, due to the need to separate the metal from the carrier.
In contrast to the teachings that suggest the need for a catalyst support, International Application WO 99/35082 discloses a process for enhancing hydrogen or carbon monoxide production in a partial oxidation reaction by feeding H2O or CO2 with the feed hydrocarbon and oxygen over a transition metal monolith catalyst such as an unsupported nickel monolith. The metal monolith may be prepared as metal foam or as sintered particles of metal. Nonetheless, it is disclosed that in some applications the metal coated ceramics will be the catalyst of choice. Furthermore, it was found that fresh nickel spheres were more difficult to ignite.
Notwithstanding the foregoing patents and teachings, there remains a need for a process for the partial oxidation of hydrocarbons using an economical unsupported catalyst in a form that is particularly suited to produce synthesis gas with high conversions of methane, with high selectivities to CO and H2, and with high gas throughput.
The present invention provides a process for the catalytic partial oxidation of a hydrocarbon feedstock by contacting a feed stream comprising the hydrocarbon feedstock and an oxygen-containing gas with a catalyst in a reaction zone maintained at conversion-promoting conditions effective to produce an effluent stream comprising carbon monoxide and hydrogen. The preferred catalyst is an unsupported monolithic catalyst containing rhodium, and is preferably rhodium foam. The preferred foam has a pore content of 75-90% by volume. Further, the preferred foam has a pore size of 20-100 ppi.
The catalyst is preferably pretreated to activate the catalyst by oxidation in air at a temperature of between about 300 and about 1200xc2x0 C., preferably between about 900 and about 1000xc2x0 C. Preferably, the air oxidation pretreatment is carried out for a period of time of about one half to four hours.
The preferred catalysts allow effective partial oxidation of the feed gas with high selectivity, together with high methane conversion. CO and H2 selectivities of at least 90% and methane conversion rates of at least 90% can be achieved. Further, the preferred catalysts have long-lived activity, with a half-life of at least six months.
The preferred operating conditions for using the present catalysts include temperatures of about 900xc2x0 C. to about 1300xc2x0 C., and more preferably from about 1000xc2x0 C. to about 1200xc2x0 C., and pressures of about 110 kPa to about 3000 kPa, and more preferably from about 500 kPa to about 3000 kPa. The feed stream comprising a hydrocarbon feedstock and oxygen gas is preferably preheated to from about 50xc2x0 C. to about 700xc2x0 C., and more preferably to about 400xc2x0 C., before contacting the catalyst. Typical space velocities for the process are from about 60,000 hrxe2x88x921 to about 20,000 hrxe2x88x921, and are preferably from about 150,000 hrxe2x88x921 to about 10,000,000 hrxe2x88x921.
Additionally, a process for the catalytic partial oxidation of a hydrocarbon feedstock is provided comprising: (a) preheating a feed stream comprising a hydrocarbon feedstock and oxygen gas to at least about 350xc2x0 C.; (b) passing the feed stream over an unsupported rhodium foam catalyst, at a space velocity from about 150,000 to about 10,000,000 NL/kg/h at conversion-promoting conditions comprising a pressure from about 500 kPa to about 2800 kPa wherein the conditions are effective to produce an effluent stream comprising carbon monoxide and hydrogen; wherein the carbon selectivity for carbon monoxide is at least about 80% and the hydrogen selectivity is at least about 60%.